Try and aliase an extra field for each Union:
SELECT tblLinkedMasterData.[Project name], tblLinkedMasterData.[Segment Project manager name], tblLinkedMasterData.[PM Management], tblLinkedMasterData.[Program Name1], tblLinkedMasterData.[RFD Initial Date], "Initial" As Gate
FROM tblLinkedMasterData
WHERE (((tblLinkedMasterData.[RFD Initial Date])>=Now() And (tblLinkedMasterData.[RFD Initial Date])<=Now()+14))
UNION
SELECT tblLinkedMasterData.[Project name], tblLinkedMasterData.[Segment Project manager name], tblLinkedMasterData.[PM Management], tblLinkedMasterData.[Program Name1], tblLinkedMasterData.[RFD Revised Date], "Revised" As Gate
FROM tblLinkedMasterData
WHERE (((tblLinkedMasterData.[RFD Revised Date])>=Now() And (tblLinkedMasterData.[RFD Revised Date])<=Now()+14)
JR